Certain conventional methods for detecting a closing time of a valve include impressing an electrical voltage into the field coil for a limited time in order to generate a field current through said field coil, as a result of which the movable armature is attracted so as to form an armature movement and lifts the valve element out of its valve seat, allowing the field current to decay to zero, then sampling and reading out the voltage profile across the field coil with respect to time.
Methods of this kind are used, for example, for determining the closing time point of injection valves for internal combustion engines. Knowledge of the duration of the valve opening process and therefore of the closing time point of the valve is an important measurement variable for controlling the quantity of fuel injected per injection operation. The closing time point of a valve can depend, for example, on the temperature, the aging state of the valve, the pressure of the fluid or gas flowing through the valve, and the throughflow rate. As a result, inaccuracies can occur when metering fluid. In order to correct these influences using a superordinate control unit, it is necessary to monitor the closing time point during operation.
In known injection valves, for example for common-rail systems, the valve element in the form of a nozzle needle rests in a valve seat when the valve is closed and thereby prevents pressurized, liquid, gaseous or liquid/gaseous fuel from flowing into the combustion space of an internal combustion engine. In order to open the valve, the valve element is lifted out of the valve seat under the action of force of an actuator and of the applied fuel pressure, against a spring force of a valve spring. Therefore, fuel can flow through between the valve element and the valve seat. In this case, the actuator is, for example, a coil drive with a movable armature which exerts the necessary force onto the valve element by virtue of an armature movement.
In order to close the valve, the valve element is pushed back into its valve seat under the action of force of the applied fuel pressure and of the valve spring, and the valve is therefore closed. It is generally not necessary for the actuator to apply force to the valve element for this purpose. Therefore, it is generally sufficient to terminate the process of impressing a voltage into the field coil in order to close the valve, with the result that, after a decay time, the field current in the coil returns to zero and accordingly the magnetic attraction to the movable armature declines. Consequently, said movable armature can fall back into its inoperative position. The reduction in current can also be accelerated by a back-e.m.f.
In systems in which the armature is fixedly coupled to the valve element, said armature, when it falls back, crashes back into its inoperative position at the same time as the valve element. In other variants, in which the armature is not fixedly coupled to the valve element, the armature can swing out freely in its inoperative position, while the valve element crashes into its seat.
Once the field current in the coil has fallen to zero, a characteristic terminal voltage, the so-called decay voltage, can be measured across the coil. This terminal voltage is based on self-induction in the coil on account of the change in magnetic field due to the falling field current and due to eddy currents which are induced in the moving armature. In ideal systems, a characteristic bend should be observed in the profile of the decay voltage at the time point which corresponds to the armature falling back into its inoperative position. This time point in turn corresponds to the closing time point of the valve, wherein the time point may be subject to a system-dependent constant offset.
Known methods for identifying the closing time point therefore use a comparison of the measured decay voltage with a reference voltage profile or derivatives of said reference voltage profile with respect to time. The closing time point can then be determined from the deviation between the two voltage profiles.
In real systems however, the profile of the decay voltage is often subject to a large amount of noise which is further amplified by derivation. In systems with an armature which swings out freely, the characteristic bend is furthermore only very slightly pronounced and it is virtually no longer possible to detect said characteristic bend using the known methods. Therefore, reliable detection of the closing time point is generally not possible in systems of this kind using the known methods.